1. Field of the Invention
This invention relates to pipelines assemblies formed by a heat-generating pipe utilizing skin-effect-current (which will be hereinafter referred to as a SECT pipe) and a fluid transporting pipe heated by the former (i.e. SECT pipe) to maintain the temperature of the latter. More particularly, it relates to the pipeline assemblies extending to a great length that requires a high voltage, e.g. exceeding 5 kV to be applied to the SECT pipe. The purpose of the present invention is to provide the economical heating assemblies for the cases where the use of insulated electric wire withstanding such high voltage is not economical or where power cannot be supplied to the SECT pipe at intermediate positions along its length, such as for underground pipelines, particularly to be laid on the seabed over a long distance.
2. Description of the Prior Art
The heat generating pipe which utilizes skin-effect-current (which will be referred to in this specification as SECT pipe) means those containing a ferromagnetic pipe, an insulated electric wire passed therethrough and such a circuit in which when a first alternating current is caused to flow through the above-mentioned electric wire, a second alternating current is caused to flow in the opposite direction in response to the first in such a manner that flow is concentrated on the vicinity of the inner surface of the ferromagnetic pipe. There are two kinds of circuit possessed by such a SECT pipe, when roughly classified. In the circuit of the first kind, the above-mentioned ferromagnetic pipe and insulated electric wire passed therethrough are electrically connected to an electric source at the respective one ends thereof and also electrically connected to each other at the respective other ends. An example of the SECT pipe having such a circuit is disclosed in the specification of Japanese Pat. No. 460,224 (or U.S. Pat. No. 3,293,407). Although the SECT pipe referred to in the above-mentioned specification requires a definite limitation to the thickness of the ferromagnetic pipe, it is possible according to the present invention to alleviate such a limitation. In the circuit of the second kind, both the ends of the above-mentioned insulated electric wire are electrically connected to an alternating current electric source to form a closed circuit and both the ends of the above-mentioned ferromagnetic pipe are electrically connected to each other so as to give as low an impedance as possible to form a closed circuit. An example of the SECT pipe having such a circuit is disclosed in the specification of Japanese Patent No. 612,750 (or U.S. Pat. No. 3,515,837). Although the SECT pipe referred to in the specification of the above-mentioned patent requires a definite limitation to the thickness of the ferromagnetic pipe, it is possible according to the present invention to alleviate such a limitation.
In the SECT pipes, whether of the first kind or of the second, the cross-sectional shape of the ferromagnetic pipe is not limited only to a circular one, but may be e.g. a triangular or lunette one. A portion of the wall of the above-mentioned ferromagnetic pipe may be constructed by the wall of the transporting pipe of ferromagnetic material to be heated. The above-mentioned ferromagnetic pipe may have intermediate breaks where the respective adjacent ends are electrically connected to each other. Further, the cross-sectional shape of the above-mentioned ferromagnetic pipe need not be a completely closed one, but may have a shape having a slit or slits. An example of such a ferromagnetic pipe is the one having a circular cross-section but containing a slit or slits in the direction of length (whereby it is possible to reduce the heat quantity generated at the portion of the ferromagnetic pipe corresponding to the length of the slit or slits). Further, another example is a ferromagnetic pipe wherein a member of ferromagnetic material having a cross-section of an inverted V shape is placed on a transporting pipe of ferromagnetic material in such a way that the skirts of the inverted V are contacted with and stitch-welded to the surface of the transporting pipe.
Now, the principle of one of known SECT pipes will be described in detail referring to FIG. 1 of the accompanying drawings. In FIG. 1, numeral 1 shows a ferromagnetic pipe such as steel pipe and numeral 2 shows an insulated electric wire or cable passed therethrough. The electric wire 2, together with a connecting electric wire 3, connect both the ends 8 and 9 of the pipe 1 to both the ends of an alternating current electric source 4, to form an alternating current circuit.
In the SECT pipe having such a circuit, the alternating current flows in concentrated manner on the vicinity of the inner surface of the ferromagnetic pipe. Namely, the nearer to the inner surface the depth, the larger the current density there, while the remoter therefrom the depth, the smaller the current density. When the specific resistivity of the material of the ferromagnetic pipe is .rho.(.OMEGA.cm); the specific permeability thereof is .mu.; and the frequency of the electric source is f (Hz), the skin depth S (cm) which is regarded as an index indicating the range in which the alternating current flows is expressed by the following equation: ##EQU1##
Further, if the thickness of the ferromagnetic pipe is t (cm), its length is l (cm) and its inner diameter is d (cm), and among them, there are relationships of EQU t&gt;2s (2); l&gt;&gt;d (3),
then the current 5 flowing through the insulated electric wire or cable 2 does not flow uniformly through the cross-section of the ferromagnetic pipe 1, but flows in concentrated manner mostly on the inner skin portion having a depth expressed by the above-mentioned equation (1). Thus, even when the outer portion of the pipe is short-circuited with a good conductor such as metal, the current 5 practically does not flow out to this short-circuiting conductor, and the current 5' flowing through the inner skin portion of the ferromagnetic pipe is nearly equal to the current 5. Even when such a SECT pipe is connected by welding to a transporting pipe of metal 12 over the total length thereof as shown in FIG. 1, the current 7 flowing out to the transporting pipe 12 is extremely small.
Now, if a commercial frequency of 60 Hz or 50 Hz is employed for the electric source and a usual steel pipe is employed as the ferromagnetic pipe, the skin depth expressed by the equation (1) is about 1 mm. Thus, a thickness of the pipe 1 of 3-5 mm can satisfy the above-mentioned equation (2), and current flows in concentrated manner on the inner thin skin portion of the steel pipe 1 where heat is generated; hence current substantially does not flow out to the outer portion of the pipe whereby safety can be maintained.
If such a SECT pipe has an inner diameter of 10.about.50 mm, a current of 50-250 A is usually employed. Thus, since the voltage per 1 km amounts to about 500 V, a pipeline having a length of about 10 km requires 5 KV in average as the electric source voltage. Thus, if electricity is fed only through both the ends of the transporting pipe, a heatable length in total, of the pipeline amounts to 20 km which is twice the above-mentioned 10 km.
Accordingly, if the distance of feed of electricity is 20 km or longer, an insulated cable 2 which endures 5 KV or higher is necessary. It is also necessary that such an insulated cable 2 endure a temperature at which the transporting pipe is maintained. Since such a temperature at which the pipe is maintained is, in general, higher than the atmospheric temperature, usual insulated cables cannot be employed and special heat-resistant, insulating materials are employed. This is expensive and thus uneconomical.
Recently, pipelines whose temperature is maintained by employing such a SECT pipe has been increasing in the length, and those exceeding 100 km are not rare. In case of such long pipelines installed on the ground, the above-mentioned uneconomical problem has been solved by feeding electricity to the pipelines at intermediate positions, but in case of such long pipelines installed on the seabed, since intermediate feed is impossible, the feed must rely only on the insulated wire pulled into the SECT pipe at its end and passed throughout the pipe, which wire, however requires a high voltage due to the long distance of the pipeline. This raises extremely uneconomical or technically infeasible problems.
The object of the present invention is to solve the difficulties in such a case.